1. Field of the Invention
The present invention relates to a sublimation thermal transfer recording method and a recording material useful for thermal printers, copying machines, facsimiles and the like.
2. Discussion of Background
Recently, the demand for full color printing is increasing year by year. There have been known various recording methods for full color printing including electrophotographic recording methods, ink jet recording methods and thermal transfer recording methods. Among these methods, the thermal transfer recording methods are widely employed because of advantages such as good image quality, high speed printing, and easy operation and maintenance.
The thermal transfer recording methods can be broadly classified into two types, a thermofusing thermal transfer recording and a sublimation thermal transfer recording.
In the thermofusing thermal transfer recording, an image can be obtained on an image receiving material upon application of heat to the backside of a thermofusing thermal transfer recording material whose ink layer contacts the image receiving material. The thermofusing thermal transfer recording material comprises a substrate such as polyester film, and an ink layer which is formed on the substrate and includes a coloring agent dispersed in a thermofusible material. When the thermal transfer recording material is subjected to the application of heat, the ink layer melts and transfers to the image receiving material, so that an image is formed on the receiving material.
Similarly, in the sublimation thermal transfer recording, an image can be obtained on an image receiving material upon application of heat to the backside of a sublimation thermal transfer recording material whose ink layer contacts a dye receiving layer of the image receiving material. The sublimation thermal transfer recording material comprises a substrate and an ink layer which is formed on the substrate and includes a thermo-diffusional dye (hereinafter referred to as a sublimable dye) dispersed in a binder resin. When the sublimation thermal transfer recording material is subjected to the application of heat, the sublimable dye diffuses into the dye receiving layer of the image receiving material, so that an image is formed on the image receiving material.
When these two recording methods are compared for full color printing, the sublimation thermal transfer recording is superior to the thermofusing thermal transfer recording with respect to fidelity of color tone and half tone and resolution of the printed image. The image formed by the sublimation thermal transfer recording has excellent image qualities as good as photographic images.
However, the sublimation thermal transfer recording method costs more to run than these other methods, because:
(a) a sublimable dye is relatively expensive; PA1 (b) yellow, magenta, cyan, and, when necessary, black image transfer recording materials, each individually being of equal size to the recorded image, are needed to obtain a full color image; and PA1 (c) a used sublimation thermal transfer recording material must be disposed of even though there is a large unused part of the recorded sheet.
To obviate this shortcoming, so-called multiple sublimation thermal transfer recording methods have been proposed. The multiple sublimation thermal transfer recording methods include the n-times (n is at least 2) mode multiple recording method and the n-fold (n is at least 2 and is generally from 5 to 20) speed mode multiple recording method.
In the n-times mode multiple recording method, a sublimation thermal transfer recording material is repeatedly printed n-times under the condition of the same feeding speed as the image receiving material.
In the n-fold speed mode multiple recording method by contrast, a sublimation thermal transfer recording material is printed under the condition of the feeding speed of 1/n to the image receiving material.
The image printed by the n-fold speed mode multiple recording method is superior to an image printed by the n-times mode multiple recording method because of advantages such as satisfactory evenness of the printed image and no wrinkling of the recording material in printing.
In general, thermal transfer recording materials, particularly sublimation thermal transfer recording materials (hereinafter referred to as recording sheets) have a heat resistant layer which includes a heat resistant resin and is formed on the opposite side of an ink layer to prevent the recording material from sticking to a thermal printhead which is a popular printing medium of thermal printing apparatus.
A variety of heat resistant layers have been proposed for thermal transfer recording materials. For example, a heat resistant layer including a heat resistant resin (Japanese Laid-Open Patent Application No. 55-7467), a heat resistant layer including a heat resistant filler to decrease friction force between a recording material and a thermal printhead (Japanese Laid-Open Patent Application No. 56-155794), and a heat resistant layer including a reaction product of polyvinylbutyral and an isocyanate, and a lubricant such as, an alkali metal salt of a phosphoric acid ester (Japanese Laid-Open Patent Application No. 61-14992), are proposed. Japanese Laid-Open Patent Applications Nos. 63-145088 and 3-65396 propose a heat resistant layer including a binder resin and a spherical filler to decrease contacting areas of the recording material to a thermal printhead.
Recently, as the demand for the sublimation thermal transfer recording continues to grow for high speed recording and/or high-n-fold speed mode multiple recording, the sublimation thermal transfer recording materials are required to be more heat resistant than before because the recording materials receive relatively high printing heat.
Therefore, such conventional heat resistant layers that include only a heat resistant resin cannot be employed for these purposes because the recording material sticks to a thermal printhead (hereinafter sticking), resulting in mis-feeding and breakage of the recording materials and occurrence of uneven printed images.
When a heat resistant layer including a filler is used and the diameter of the filler becomes too small, the heat resistant layer cannot prevent sticking because the contacting areas of the heat resistant layer to a thermal printhead cannot be decreased. On the other hand, when the diameter of a filler is large enough to prevent sticking, another problem occurs such that undesirable white spots appear in a printed image, particularly in a half tone image. The reason for the problem is that the heat energy received by the ink layer near the large particles of the filler in the heat resistant layer is not enough to form an image because of the insufficient contact between the heat resistant layer near the large particles of the filler and the thermal printhead. In the n-fold speed mode multiple recording, the length of the white spots are magnified n-times in the feeding direction of the receiving material, so that the image quality is seriously deteriorated.
In the n-fold speed mode multiple recording, since the recording material runs more slowly than in normal sublimation thermal transfer recording (n=1), the recording material receives relatively high printing heat. Therefore, the recording material used in the n-fold speed mode multiple recording is required to have excellent heat resistance.
In addition, in the n-fold speed mode multiple recording, since the recording material and the image receiving material are run at different speed in printing, a strong friction force occurs between the recording material and the image receiving material, so that problems such as, sticking, and thus, uneven printed images resulting therefrom, tend to occur more frequently than normal sublimation thermal transfer recording.
Further, when using a crosslinkable resin in a heat resistant layer to improve heat resistance property, problems occur such as instability of heat resistance property caused by uneven crosslinking of the heat resistant layer and gelation of the coating liquid during the coating process of the heat resistant layer.
Because of these reasons, a need exists for a sublimation thermal transfer recording method and a recording material therefor in which a printed image has excellent image qualities without uneven image caused by sticking and undesirable white spot images even when used for an n-fold speed mode multiple recording.